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21.1.2 Session and Subsession
Subsession
"One subsession" refers to transfer of one parameter set. A subsession transfers one parameter set or a parameter set that
has been divided into multiple packets for transfer, with EOD (End of Data) at the end to terminate the send.
Division of a parameter set into multiple packets is used when the size of the parameter set is greater than a prescribed
size. The packet number in the packet index field indicates the sequential position of a packet relative to the other packets.
A single packet cannot be used to transfer multiple small parameter sets. A parameter set delimiter always must be transferred
as a packet delimiter.
Session
"One session" refers to a series of processes that occur for one user operation. One subsession or multiple subsessions
make up a session. The sender sends EOS (End of Session) to end a session.
Regardless of whether there is a single parameter set or multiple parameter sets being transferred, a bulk dump always
takes the form of a session, never a subsession only.
21.2 One-way Mode Communication Flow
A session starts with the receiving device sending a request using an OBR, or with the sending device sending OBS data.
The session ends after transfer of all the data in the parameter set being transferred by the sending device is complete.
The transferred messages of a single parameter set cannot exceed 256 bytes. When 256 bytes are exceeded, data is divided
into multiple packets of 256 bytes or less each and transferred at fixed intervals (20 msec).
A final EOD informs the receiving device when the session is ended.
Data Receiver
(External Device)
Data Sender
(This Instrument)
Operation
OBR → Send Request (Optional)
← OBS
(20 msec or greater interval)
Data Transfer
← OBS
(20 msec or greater interval)
Data Transfer
← OBS
(20 msec or greater interval)
Data Transfer
:
:
← EOD End of Data
:
Other subsessions
:
← EOS End of Session